1. Field of the Invention
The present invention relates to novel borate esters made from 2,2'-thiobis(alkylphenols) and thiobis(alkylnaphthols) and to various hydrocarbyl compositions, e.g., lubricants, which are subject to oxidative degradation, containing a minor amount of said borate esters, sufficient to impart antioxidant characteristics thereto. More particularly, the present invention is directed to the aforedescribed novel borate esters and lubricant compositions containing same. The lubricant compositions in accordance herewith include various hydrocarbyl oils of lubricating viscosity, including hydrocracked lubricating oils, and oils adapted for use as hydraulic oils, automotive oils, gear oils, transmission fluids and waxes, greases and other solid forms prepared therefrom.
2. Description of the Prior Art
The employment of borate esters as antioxidants in lubricants has been known in the prior art. However, a serious drawback which has restricted the utility of borate esters generally and aryl borate esters in particular in lubricants has been the susceptibility to hydrolysis of such esters with the concomitant liberation of oil-insoluble, abrasive boric acid and simultaneous loss of desirable boron-imparted properties to the lubricants. U.S. Pat. No. 3,356,707 discloses hydrolytically stable borate esters of hindered phenols. However, it has been now found that it is not essential to have a hindered phenol ester group in the compositions of the present invention to achieve hydrolytic stability. Nevertheless, borate esters of certain hindered phenols can be highly useful.